1. Field of the Invention
This invention relates to a fuel consumption measuring device used for measuring a fuel consumption of an engine and, more particularly, to a fuel consumption measuring device suitable for an engine having a return fuel, such as an electronic fuel injection gasoline engine or a diesel engine.
2. Description of the Prior Art
FIG. 5 is a system diagram showing one embodiment of a conventional fuel consumption measuring device.
In a conventional fuel consumption measuring device 50, fuel is pumped out of a fuel tank 61 by a pump 62, and has a predetermined pressure which is set by a relief valve 51. Further, the relief valve 51 is connected to a flow meter 53 through a filter 52 or the like.
The flow meter 53 may be provided with opposing four pistons and cylinders, and crankshafts. The fuel is fed through an intermediate portion on which crankshafts are mounted, and is exhausted after passing through four circumferential ports. At this time, these four pistons serve as a three way valve with respect to the adjacent pistons in a counterclockwise manner. The respective pistons are alternately operated to fill the respective cylinders with the fuel, and to exhaust the fuel out of the cylinders. The piston is reciprocated to rotate the crankshaft which is provided with a permanent magnet. Accordingly, a rotation detector magnetically coupled with the crankshaft can be driven to fetch a pulse signal corresponding to a flow rate.
The fuel having passed through the flow meter 53 is once accumulated in a return processing tank 54, and is supplied by a pump 63 to injectors 64. The fuel is then injected to an inlet manifold portion of an engine (not shown).
Fuel injection quantity is determined by a valve opening time of the injector 64 and a differential pressure between pressure of a fuel line (fuel feeding passage) and pressure of the inlet manifold. Therefore, it is necessary to always hold a constant differential pressure between the pressure of the fuel line and the pressure of the inlet manifold.
Hence, a primary pressure regulator 65 is operated to return excess fuel through a return pipe into the return processing tank 54 so as to hold a constant pressure in the fuel feeding passage.
The return processing tank 54 is a device used for processing a return fuel, and is communicated with atmosphere to be held at atmospheric pressure.
The return processing tank 54 is provided with a float valve 55 which controls to maintain a constant liquid level. In the float valve 55, when the liquid level is lowered to lower the float, a needle of the valve is lowered to cause inflow of the fuel.
Thus, it is possible to constantly supply the same quantity of the fuel as that of the fuel consumed by an actual injection of the injectors 65 to the return processing tank 54. Since the flow meter 54 is operated to measure the quantity of the supplied fuel, the fuel consumption in the injectors 65 can be measured.
A heat exchanger 56 is provided to cool the fuel returned into the return processing tank 54 by using the fuel in the fuel tank 61 when the temperature of the returned fuel is increased.
The conventional fuel consumption measuring device as described above is operated to measure the fuel consumption based on the inflow rate of the fuel into the return processing tank 54 while controlling to maintain the constant liquid level by the float valve 55. Consequently, since the liquid level position is varied when vibration is generated or when the flow rate is varied, it is impossible to measure the fuel consumption accurately, in particular, during measuring in the transient state. Further, the conventional fuel consumption measuring device can not be used as a vehicle equipment because it is too sensitive to the vibration as described hereinbefore.
In addition, a pipe is in the opened condition since the return processing tank 54 should be opened to atmosphere. Accordingly, there is a problem of a low security of the return processing tank 54.